Selective para-xylene production via methylation of toluene with methanol in the presence of modified HZSM-5 catalyst

ABSTRACT

We disclose a method for converting toluene to xylenes, comprising contacting toluene with methanol in the presence of a silica-bound HZSM-5 catalyst. As an example, in one embodiment the method can include: (i) first silylating HZSM-5, to form silylated HZSM-5; (ii) first calcining the silylated HZSM-5, to form calcined silylated HZSM-5; (iii) binding the calcined silylated HZSM-5 to silica, to form silica-bound calcined silylated HZSM-5; (iv) extruding the silica-bound calcined silylated HZSM-5, to form extruded silica-bound calcined silylated HZSM-5; (v) second calcining the extruded silica-bound calcined silylated HZSM-5, to form extruded silica-bound twice-calcined silylated HZSM-5; (vi) second silylating the extruded silica-bound twice-calcined silylated HZSM-5, to form extruded silica-bound twice-calcined twice-silylated HZSM-5; and (vii) third calcining the extruded silica-bound twice-calcined twice-silylated HZSM-5, to form the silica-bound HZSM-5 catalyst.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of xylenesproduction. More particularly, it concerns methods for convertingtoluene to xylenes using catalyst systems capable of producing p-xylenein high yield.

Methylation of toluene with methanol is known in the art as a usefultechnique for the formation of xylenes. The reaction has the followinggeneral formula (Formula I):

wherein x, y, and z are each between 0 and 1, inclusive, and x+y+z=1.

From top to bottom on the right of Formula I are ortho-xylene (o-xylene,1,2-dimethylbenzene), meta-xylene (m-xylene, 1,3-dimethylbenzene), andpara-xylene (p-xylene, 1,4-dimethylbenzene). Of the three, p-xylene isparticularly useful in making, either directly or by substitution at themethylene moieties, straight-chain polymers such as polyethyleneterephthalate (PET). Generally, however, the methylation of tolueneproduces all three of the xylene isomers shown in Formula I. Althoughthe separation of p-xylene from a raw product mixture containing theother xylene isomers is possible, it will be apparent that techniquesfor maximizing the fraction of the raw product mixture defined byp-xylene (i.e., increasing z in Formula I) are beneficial.

SUMMARY OF THE INVENTION

In one embodiment, the present invention relates to a method forconverting toluene to xylenes, comprising contacting toluene withmethanol in the presence of a silica-bound HZSM-5 catalyst.

In another embodiment, the present invention relates to a method forconverting toluene to xylenes, comprising contacting toluene withmethanol in the presence of a silica-bound HZSM-5 catalyst, wherein thecatalyst can be prepared by a process comprising: (i) first silylatingHZSM-5, to form silylated HZSM-5; (ii) first calcining the silylatedHZSM-5, to form calcined silylated HZSM-5; (iii) binding the calcinedsilylated HZSM-5 to silica, to form silica-bound calcined silylatedHZSM-5; (iv) extruding the silica-bound calcined silylated HZSM-5, toform extruded silica-bound calcined silylated HZSM-5; (v) secondcalcining the extruded silica-bound calcined silylated HZSM-5, to formextruded silica-bound twice-calcined silylated HZSM-5; (vi) secondsilylating the extruded silica-bound twice-calcined silylated HZSM-5, toform extruded silica-bound twice-calcined twice-silylated HZSM-5; and(vii) third calcining the extruded silica-bound twice-calcinedtwice-silylated HZSM-5, to form the silica-bound HZSM-5 catalyst.

In a further embodiment, the present invention relates to a method forconverting toluene to xylenes, comprising contacting toluene withmethanol in the presence of a silica-bound HZSM-5 catalyst, wherein thecatalyst can be prepared by a process comprising: (i) binding HZSM-5 tosilica, to form silica-bound HZSM-5; (ii) silylating the silica-boundHZSM-5, to form silylated silica-bound HZSM-5; (iii) calcining thesilylated silica-bound HZSM-5, to form calcined silylated silica-boundHZSM-5; (iv) first washing the calcined silylated silica-bound HZSM-5,to form washed calcined silylated silica-bound HZSM-5; (v) first dryingthe washed calcined silylated silica-bound HZSM-5, to form driedcalcined silylated silica-bound HZSM-5; (vi) steaming the dried calcinedsilylated silica-bound HZSM-5, to form steamed calcined silylatedsilica-bound HZSM-5; (vii) treating the steamed calcined silylatedsilica-bound HZSM-5 with acid, to form acid-treated calcined silylatedsilica-bound HZSM-5; (viii) second washing the acid-treated calcinedsilylated silica-bound HZSM-5, to form washed acid-treated calcinedsilylated silica-bound HZSM-5; and (ix) second drying the washedacid-treated calcined silylated silica-bound HZSM-5, to form thesilica-bound HZSM-5 catalyst.

Other aspects and features of the invention will become apparent inlight of the detailed description and the claims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In one embodiment, the present invention relates to a method forconverting toluene to xylenes, comprising: (a) contacting toluene withmethanol in the presence of a silica-bound HZSM-5 catalyst. The catalystof the present invention can be prepared by a number of processes, threeof which will be described below.

Techniques for contacting toluene with methanol in the presence of asolid catalyst for the production of xylenes are well known in the art.(The term “xylenes,” when used without an isomer-designating prefix,refers herein to a mixture of ortho-, meta-, and para-xylenes). Any of anumber of well known methods can be used for producing xylenes in lightof the present disclosure, and the invention is not limited by theparticular method and reaction parameters used.

HZSM-5 is known in the art. ZSM-5 is an aluminosilicate zeolite with ahigh silica content and a low aluminum content. Its structure can beconsidered to comprise channels with intersecting tunnels. The aluminumsites are generally acidic, due to the substitution of Al³⁺ in place oftetrahedral Si⁴⁺ silica requiring the presence of an added positivecharge. When this positive charge is provided by H⁺, the zeolite istermed HZSM-5.

HZSM-5 is commercially available from Süd-Chemie Inc., Louisville, Ky.,among other sources.

In one embodiment, the catalyst can be prepared by a process comprising(i) binding the HZSM-5 to silica, to form the silica-bound HZSM-5.“Binding” in this context, as used herein, generally refers tophysically admixing HZSM-5 and silica in such a manner that the silicaprovides a support for the HZSM-5. The proportion of HZSM-5 in theadmixture, as a weight percentage of HZSM-5 and silica, can be fromabout 1 wt % to about 90 wt %. In one embodiment, the proportion ofHZSM-5 in the admixture can be from about 2 wt % to about 50 wt %.

Silica is well known in the art and commercially available from a numberof suppliers.

The silica-bound HZSM-5 catalyst prepared according to this embodimentcan catalyze the methylation of toluene to xylenes with a para-xyleneselectivity in the range of about 65% to 100% of all xylenes produced(the percentages can be interchangeably expressed by mole fraction or bymass).

In another embodiment, the present invention relates to a method forconverting toluene to xylenes, comprising:

(a) contacting toluene with methanol in the presence of a silica-boundHZSM-5 catalyst, wherein the catalyst can be prepared by a processcomprising: (i) first silylating HZSM-5, to form silylated HZSM-5; (ii)first calcining the silylated HZSM-5, to form calcined silylated HZSM-5;(iii) binding the calcined silylated HZSM-5 to silica, to formsilica-bound calcined silylated HZSM-5; (iv) extruding the silica-boundcalcined silylated HZSM-5, to form extruded silica-bound calcinedsilylated HZSM-5; (v) second calcining the extruded silica-boundcalcined silylated HZSM-5, to form extruded silica-bound twice-calcinedsilylated HZSM-5; (vi) second silylating the extruded silica-boundtwice-calcined silylated HZSM-5, to form extruded silica-boundtwice-calcined twice-silylated HZSM-5; and (vii) third calcining theextruded silica-bound twice-calcined twice-silylated HZSM-5, to form thesilica-bound HZSM-5 catalyst.

The HZSM-5 is as described above.

In the first silylating step, the HZSM-5 can be silylated to formsilylated HZSM-5.

“Silylating,” as used herein, refers to the treating of HZSM-5 withsilicon compounds selected from the group of alkoxysilanes andpolysiloxanes.

An alkoxysilane consists of compounds having the formulaSi(R′_(n))(OR)_((4−n)), wherein n is an integer from 0-3, inclusive,each R and R′ is independently a C₁-C₆ alkyl group or a phenyl group.Such alkoxysilanes can be tetramethoxysilane, tetraethoxysilane,tetrapropoxysilane, tetrabutoxysilane, tetraisopropoxysilane,tetraisobutoxysilane and tetrasecbutoxysilane. Exemplary alkoxysilanesinclude, but are not limited to, tetraethoxysilane (TEOS), among others.

A polysiloxane consists of compounds having the formulaSi(R′_(x))(OR)_((3−x))[Si(R′_(y))(OR)_((2−y))]_(n)OSi(R′_(z))(OR)_((3−z)),wherein x and z are an integer from 0-3, inclusive, y is an integer from0-2, inclusive, n is the degree of polymerization, and each R and R′ isindependently a C₁-C₆ alkyl group or a phenyl group. Polysiloxanesconsist of organosubstituted polysiloxane compounds of varying degreesof polymerization. The only requirement regarding degrees ofpolymerization, for the instant process, is that the viscosity of thefluid be such that the HZSM-5 can be adequately mixed with thepolysiloxane to insure good contact.

The polysiloxanes can be totally methylated fluids or methyl and phenylsubstituted fluids. The polysiloxanes can be either linear or cyclic instructure. The polysiloxanes can be dimethyl and methylphenylsubstituted copolymers.

The polysiloxanes can be, for example, trimethylsilyl end-blocked methyland phenyl substituted polysiloxanes, trimethylsilyl end-blockeddimethylpolysiloxanes, methylphenylhydroxysilyl end-blocked methyl andphenyl substituted polysiloxanes dimethylcyclopolysiloxanes, and methyland phenyl substituted cyclopolysiloxanes. Exemplary polysiloxanesinclude, but are not limited to poly(methyl phenyl)siloxane (PMPS),among others.

In a silylating step, the HZSM-5 can be contacted with the alkoxysilaneaccording to any appropriate technique, such as a batch or continuousprocess; with the alkoxysilane in the vapor, liquid, or solid phase;among others. If the alkoxysilane is provided in the vapor phase, acarrier or diluent gas, such as nitrogen, helium, argon, carbon dioxide,air, or steam, among others, can be used. The alkoxysilane concentrationin the gas (as vol % relative to the total volume of the gases) can befrom about 0.1 vol % to 100 vol %. If the alkoxysilane is provided inthe liquid phase, a solvent, such as benzene, toluene, xylenes, pentane,hexane, heptane, octane, methanol, ethanol, or propanol, among others,can be used. The alkoxysilane concentration in the solution (as wt %relative to the total weight of the gases) can be from about 0.1 wt % toabout 10 wt %. Selecting a particular diluent gas or solvent is a matterof routine experimentation for the skilled artisan.

A silylating step can be performed at any suitable temperature and forany suitable duration. A suitable temperature for silylating with gasphase alkoxysilane can be from about 10° C. to about 600° C., and withliquid phase alkoxysilane, from about 10° C. to about 150° C. Theduration of silylating can be up to about 1 hr, up to about 2 hr, up toabout 3 hr, up to about 6 hr, up to about 12 hr, up to about 18 hr, upto about 24 hr, up to about 36 hr, up to about 48 hr, up to about 72 hr,or more than about 72 hr. In one embodiment, the duration of silylatingis at least about 6 hr.

The amount of the alkoxysilane used to silylate the HZSM-5 can vary. Inone embodiment, the amount of the alkoxysilane (on a weight basis ofsilicon to the weight of the HZSM-5) to be introduced to the HZSM-5 canbe from about 1 wt % to about 10 wt % (i.e., from about 1 g Si asalkoxysilane per 100 g HZSM-5 to about 10 g Si as alkoxysilane per 100 gHZSM-5).

In one embodiment, the first silylating step comprises silylating theHZSM-5 with tetraethoxysilane (TEOS).

The product of the first silylating step is a silylated HZSM-5.

In the first calcining step, the silylated HZSM-5 can be calcined toform calcined silylated HZSM-5.

“Calcining,” as used herein, refers to the treatment of a material withheat under an oxidizing environment. The amount of heat applied can beup to any relatively high temperature, such as about 200° C. or higher,about 300° C. or higher, about 400° C. or higher, about 500° C. orhigher, or about 600° C. or higher. In one embodiment, calcining isperformed at a temperature from about 500° C. to about 600° C. An“oxidizing environment” is any environment, typically a gas, under whichcarbon atoms present in the material can be reacted with compounds inthe environment to form carbon dioxide. Typically, the oxidizingenvironment comprises gaseous oxygen or gaseous air. The duration of thecalcining step is not critical and can be adjusted by the skilledartisan having the benefit of the present disclosure.

The product of the first calcining step is calcined silylated HZSM-5.

In the binding step, the calcined silylated HZSM-5 can be bound tosilica, to form silica-bound calcined silylated HZSM-5. The binding stepcan be performed as described above, with the portions of calcinedsilylated HZSM-5 and silica described above.

In the extruding step, the silica-bound calcined silylated HZSM-5 can beextruded to form extruded silica-bound calcined silylated HZSM-5.

“Extruding,” as used herein, involves the molding of a liquid, powder,or otherwise moldable material into a rigid solid form. Extruding caninvolve the use of heat or pressure. The term “extruding” encompassesboth batchwise molding techniques and continuous molding techniques. Inone embodiment, the extruding step comprises producing a 1/16 inchdiameter solid extrudate and pelletizing the extrudate.

The product of the extruding step is extruded silica-bound calcinedsilylated HZSM-5.

In the second calcining step, the extruded silica-bound calcinedsilylated HZSM-5 is calcined to form extruded silica-boundtwice-calcined silylated HZSM-5. Calcining has been described above.

In the second silylating step, the extruded silica-bound twice-calcinedsilylated HZSM-5 can be silylated to form extruded silica-boundtwice-calcined twice-silylated HZSM-5. Silylating has been describedabove. In one embodiment, the second silylating step comprisessilylating the HZSM-5 with poly(methyl phenyl)siloxane (PMPS).

In the third calcining step, the extruded silica-bound twice-calcinedtwice-silylated HZSM-5 can be calcined to form the silica-bound HZSM-5catalyst. The silica-bound HZSM-5 catalyst prepared according to thisembodiment can catalyze the methylation of toluene to xylenes with apara-xylene selectivity in the range of about 65% to 100%, such as about85%, of all xylenes produced.

In a further embodiment, the present invention relates to a method forconverting toluene to xylenes, comprising (a) contacting toluene withmethanol in the presence of a silica-bound HZSM-5 catalyst, wherein thecatalyst can be prepared by a process comprising: (i) binding HZSM-5 tosilica, to form silica-bound HZSM-5; (ii) silylating the silica-boundHZSM-5, to form silylated silica-bound HZSM-5; (iii) calcining thesilylated silica-bound HZSM-5, to form calcined silylated silica-boundHZSM-5; (iv) first washing the calcined silylated silica-bound HZSM-5,to form washed calcined silylated silica-bound HZSM-5; (v) first dryingthe washed calcined silylated silica-bound HZSM-5, to form driedcalcined silylated silica-bound HZSM-5; (vi) steaming the dried calcinedsilylated silica-bound HZSM-5, to form steamed calcined silylatedsilica-bound HZSM-5; (vii) treating the steamed calcined silylatedsilica-bound HZSM-5 with acid, to form acid-treated calcined silylatedsilica-bound HZSM-5; (viii) second washing the acid-treated calcinedsilylated silica-bound HZSM-5, to form washed acid-treated calcinedsilylated silica-bound HZSM-5; and (ix) second drying the washedacid-treated calcined silylated silica-bound HZSM-5, to form thesilica-bound HZSM-5 catalyst.

In the binding step, the HZSM-5 can be bound to silica, to formsilica-bound HZSM-5. The binding of HZSM-5 to silica has been describedabove.

In the silylating step, the silica-bound HZSM-5 can be silylated to formsilylated silica-bound HZSM-5. Silylating has been described above. Anyalkoxysilane can be used. In one embodiment, the silylating stepcomprises contacting the silica-bound HZSM-5 with PMPS. The product ofthe silylating step is a silylated silica-bound HZSM-5.

In the calcining step, the silylated silica-bound HZSM-5 can be calcinedto form calcined silylated silica-bound HZSM-5. Calcining has beendescribed above.

In the first washing step, the calcined silylated silica-bound HZSM-5can be washed to form washed calcined silylated silica-bound HZSM-5.

“Washing,” as used herein, refers to the application of a liquid, suchas water, a water-surfactant solution, or an organic solvent, to amaterial. The washing can involve one or more applications of the liquidto the material, and can involve one or more soaking applicationsfollowed by one or more rinsing applications, or one or more combinedsoaking/rinsing applications, among other possibilities that willrecommend themselves to the skilled artisan having the benefit of thepresent disclosure. Washing can be performed at ambient temperature andpressure or adjusted to higher or lower temperature, higher or lowerpressure, or both.

The product of the first washing step is a washed calcined silylatedsilica-bound HZSM-5.

In the first drying step, the washed calcined silylated silica-boundHZSM-5 can be dried to form dried calcined silylated silica-boundHZSM-5.

“Drying,” as used herein subsequent to washing, refers to theevaporation, either passive or forced, of a liquid used in washing froma material. Forced evaporation can include the use of heat, forced air,or desiccant materials, among other materials or techniques.

The product of the first drying step is dried calcined silylatedsilica-bound HZSM-5.

In the steaming step, the dried calcined silylated silica-bound HZSM-5can be steamed to form steamed calcined silylated silica-bound HZSM-5.

“Steaming,” as used herein, refers to contacting a material with a gasmixture comprising water vapor and wherein the gas mixture has atemperature of greater than about 100° C., such as from about 500° C. toabout 700° C. The duration of the steaming step is not critical. Typicaldurations of the steaming step can be from about 3 hr to about 12 hr.Generally, though not to be bound by theory, the use of highertemperatures, longer durations, or both in the steaming step will leadto greater dealuminumization in the catalyst. In one embodiment, thesteaming step comprises the use of a gas mixture consisting essentiallyof water vapor at 600° C. for a duration of about 6 hr.

The product of the steaming step is a steamed calcined silylatedsilica-bound HZSM-5.

In the treating step, the steamed calcined silylated silica-bound HZSM-5can be treated with acid, to form acid-treated calcined silylatedsilica-bound HZSM-5.

“Treating with acid,” as used herein, refers to the contacting of amaterial with an acidic solution, i.e., an aqueous solution comprisingan acid and having a pH of less than 7. Any mineral or organic acid canbe used. In one embodiment, the acid can be a mineral acid. In oneembodiment, the mineral acid can be hydrochloric acid (HCl). In oneembodiment, the acidic solution comprises from about 1 wt % HCl tosaturation with HCl (about 38 wt %). In one embodiment, in the treatingstep the acid can be hydrochloric acid at about 0.1 N in aqueoussolution.

The temperature, duration, and other parameters of the treating step arenot critical. Generally, the temperature can be from about roomtemperature to about 100° C., and the duration can be from about 1 hr toabout 48 hr, such as from about 6 hr to about 24 hr. In one embodiment,the treating step is performed at about 90° C. for about 16 hr.

The product of the treating step is an acid-treated calcined silylatedsilica-bound HZSM-5.

In the second washing step, the acid-treated calcined silylatedsilica-bound HZSM-5 is washed to form washed acid-treated calcinedsilylated silica-bound HZSM-5. The washing step can be performed as isdescribed above.

In the second drying step, the washed acid-treated calcined silylatedsilica-bound HZSM-5 can be dried to form the silica-bound HZSM-5catalyst. The drying step can be performed as is described above.

The silica-bound HZSM-5 catalyst prepared according to this embodimentcan catalyze the methylation of toluene to xylenes with a para-xyleneselectivity in the range of about 65% to 100%, such as essentially 100%,of all xylenes produced.

The following examples are included to illustrate particular embodimentsof the invention. However, those of skill in the art should, in light ofthe present disclosure, appreciate that many changes can be made in thespecific embodiments which are disclosed and still obtain a like orsimilar result without departing from the spirit and scope of theinvention.

EXAMPLE 1

HZSM-5 zeolite was bound to silica (Ludox AS-40) by admixing, andextruded ( 1/16 in pellets). The extruded material was calcined (538°C., 6 hr). The silica-bound HZSM-5 (13.4 g) was silylated with 26.8 g ofa 10 wt % poly(methyl phenyl)siloxane (PMPS) solution in cyclohexane.Thereafter, the product was calcined at 538° C. for 6 hr; washed inwater; and dried (538° C., 2 hr). 13.7 g of dried calcined silylatedsilica-bound HZSM-5 were produced. An aliquot (6.0 g) of the driedcalcined silylated silica-bound HZSM-5 was then subjected to 100% steam,600° C., for 6 hr. After steaming, the composition was treated with 200g 0.1 N HCl at 90° C. for 16 hr; washed in water; and dried (538° C., 2hr). A total of 6.0 g of silica-bound HZSM-5 catalyst was prepared (as a1/16 inch extrudate).

Thereafter, the catalyst was used to catalyze the methylation of tolueneby methanol. The catalyst was charged to a reactor, and feeds commencedof 20 wt % methanol/80 wt % toluene (20 mL/hr) and hydrogen (260mL/min). The reactor was maintained at about 600° C. and a pressure ofabout 70-80 psig. The reaction was allowed to proceed for about 7.5 hr,with product quantities measured during the run by GC-FID.

Table 1 presents the percentage conversion of methanol (i.e., the weightpercentage of all methanol added to the reactor minus the weightpercentage of methanol present in the reactor), the weight percentageyield of all xylenes (i.e., the weight of all xylenes present divided bythe weight of all methanol and toluene added to the reactor, multipliedby 100%), and the selectivity of para-xylene (the weight of para-xylenedivided by the weight of all xylenes present, multiplied by 100%) atfour time points.

TABLE 1 Methanol Total xylenes yield Para-xylene Time (hr) conversion(wt %) (wt %) selectivity (wt %) 3.25 100.0 11.5 100.0 4.75 100.0 11.3100.0 6.00 100.0 10.7 100.0 7.50 100.0 10.7 100.0

Table 1 indicates that essentially complete selectivity to para-xylenein the methylation of toluene is possible by the use of a catalyst ofthe present invention.

EXAMPLE 2

HZSM-5 zeolite was silylated with a 10% solution of tetraethylorthosilicate (TEOS), and then was calcined at 538° C. for 6 hr. Thecalcined silylated HZSM-5 was then bound to silica (Ludox AS-40) byadmixing, and extruded ( 1/16 inch pellets). The extruded material wasagain calcined (538° C. for 6 hr). The calcined extruded material wassilylated with a 10 wt % poly(methyl phenyl)siloxane (PMPS) (Dow 510)solution in cyclohexane and third calcined (538° C. for 6 hr) to producethe silica-bound HZSM-5 catalyst of this example.

Thereafter, the catalyst was used to catalyze the methylation of tolueneby methanol. The catalyst was charged to a reactor, and feeds commencedof 20 wt % methanol/80 wt % toluene (20 mL/hr) and hydrogen (260mL/min). The reactor was maintained at about 600° C. and a pressure ofabout 70-86 psig. The reaction was allowed to proceed for about 7 hr,with product quantities measured during the run by GC-FID.

At the end of the reaction duration, it was discovered that the reactionexhibited methanol conversion of 99.7 wt % of added methanol; totalxylenes yield of 21.5 wt % of added methanol plus added toluene; andpara-xylene selectivity of 84.6 wt % of total xylenes.

This example indicates that methylation of toluene with use of acatalyst of the present invention can produce para-xylene with highselectivity.

All of the compositions and methods disclosed and claimed herein can bemade and executed without undue experimentation in light of the presentdisclosure. While the compositions and methods of this invention havebeen described in terms of preferred embodiments, it will be apparent tothose of skill in the art that variations may be applied to thecompositions and methods and in the steps or in the sequence of steps ofthe method described herein without departing from the concept, spiritand scope of the invention. More specifically, it will be apparent thatcertain agents which are chemically related may be substituted for theagents described herein while the same or similar results would beachieved. All such similar substitutes and modifications apparent tothose skilled in the art are deemed to be within the spirit, scope andconcept of the invention as defined by the appended claims.

1. A method for preparing a catalyst composition for converting tolueneto xylenes, comprising: (i) first silylating HZSM-5 with analkoxysilane, to form silylated HZSM-5; (ii) first calcining thesilylated HZSM-5, to form calcined silylated HZSM-5; (iii) binding thecalcined silylated HZSM-5 to silica, to form silica-bound calcinedsilylated HZSM-5; (iv) extruding the silica-bound calcined silylatedHZSM-5, to form extruded silica-bound calcined silylated HZSM-5; (v)second calcining the extruded silica-bound calcined silylated HZSM-5, toform extruded silica-bound twice-calcined silylated HZSM-5; (vi) secondsilylating the extruded silica-bound twice-calcined silylated HZSM-5, toform extruded silica-bound twice-calcined twice-silylated HZSM-5; and(vii) third calcining the extruded silica-bound twice-calcinedtwice-silylated HZSM-5, to form the silica-bound HZSM-5 catalyst.
 2. Themethod of claim 1, wherein the first silylating step comprisescontacting the HZSM-5 with tetraethoxysilane (TEOS), and the secondsilylating step comprises contacting the extruded silica-boundtwice-calcined silylated HZSM-5 with poly(methyl phenyl) siloxane(PMPS).
 3. A catalyst composition for converting toluene to xylenes,prepared by a process comprising: (i) first silylating HZSM-5 with analkoxysilane, to form silylated HZSM-5; (ii) first calcining thesilylated HZSM-5, to form calcined silylated HZSM-5; (iii) binding thecalcined silylated HZSM-5 to silica, to form silica-bound calcinedsilylated HZSM-5; (iv) extruding the silica-bound calcined silylatedHZSM-5, to form extruded silica-bound calcined silylated HZSM-5; (v)second calcining the extruded silica-bound calcined silylated HZSM-5, toform extruded silica-bound twice-calcined silylated HZSM-5; (vi) secondsilylating the extruded silica-bound twice-calcined silylated HZSM-5, toform extruded silica-bound twice-calcined twice-silylated HZSM-5; and(vii) third calcining the extruded silica-bound twice-calcinedtwice-silylated HZSM-5, to form the silica-bound HZSM-5 catalyst.
 4. Thecatalyst composition of claim 3, wherein the first silylating stepcomprises contacting the HZSM-5 with tetraethoxysilane (TEOS), and thesecond silylating step comprises contacting the extruded silica-boundtwice-calcined silylated HZSM-5 with poly(methyl phenyl) siloxane(PMPS).